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Standard Cyclopedia of Horticulture
The leaf is the primary site of photosynthesis in plants.
A Photosystem: A light-harvesting cluster of photosynthetic pigments present in the thylakoid membrane of chloroplasts.

Photosynthesis. Green plants exposed to sunlight at a growing temperature are able to manufacture organic food substances, that is, carbohydrates. The term photosynthesis, derived from Greek words signifying "light" and "putting together," is applied to this process of food manufacture. Green plants manufacture not only their own food carbohydrates but also are the sources of practically all of the organic matter which may eventually furnish food for botn plants and animals. It may be said, therefore, that life today is dependent upon the green leaf. The first carbon-containing compound made is a relatively simple substance, but the first recognizable material is sugar. The crude materials out of which organic substance is made in the cells of the green tissues are CO2 (carbon dioxide) and water. The leaf green, chlorophyl, and the protoplasm of the cell may be regarded as the important mechanism, while the source of energy for the chemical change induced is radiant energy, light. Air ordinarily contains about .03 per cent of CO2, yet the ordinary green plant obtains all of its carbon for the making of organic matter from this extremely small quantity in the atmosphere. The chlorophyl is important inasmuch as it absorbs the radiant energy which is directly or indirectly responsible for the process. Chlorophyl is distributed within the cells in definite granules, or small bodies, protoplasmic in nature, commonly ovoidal in form. The light absorbed is largely from the red or red-orange portion of the spectrum. It is possible that the energy so derived is first transformed into electrical energy, yet little is known upon this point. It is certain, however, that green plants are unable to utilize energy derived, for example, from the absorption of heat. The process may be briefly pictured in the following manner: The cell-sap absorbs the CO2 which diffuses into the tissues from the air. By means of the energy absorbed by the chlorophyl bodies, within the cells, the CO2 is supposed to be reduced to CO (carbon monoxide), and the same means resolves the water into its constituents. The products of these molecular changes form new substances, perhaps formaldehyde (CH2O) and oxygen (O2). The formation of formaldehyde is still somewhat uncertain; but in any case sugar is soon recognized. In all probability the formaldehyde molecules are immediately condensed to sugar (C6H12O6). It will be noted that the surplus oxygen is in reality a by-product and during active photosynthesis it is produced in such quantity as to be actively eliminated from the plant by diffusion. The usual test for photosynthesis is carried out by counting the bubbles given off from the cut stem of a water plant exposed to sunlight in a well-aerated vessel of spring-water. The content of oxygen in these bubbles is greater than that of normal air, and the rate of bubble-production is a fair estimate of the rate of photo-synthesis.

As a rule the sugar formed in the leaf does not accumulate to any large extent, but is transformed into starch. Some of the sugar, however, may be immediately diffused to other cells or "transported," supplying the needs of this substance in growth. The starch which is deposited is in the form of insoluble granules, and the formation of these bodies on exposure of the green leaf to sunlight is so rapid as to make it possible in some cases to use starch formation as an index to rate of photosynthesis. During the night, when no photosynthesis occurs, the transformation and removal of the starch usually goes on rapidly, so that within an interval of twelve hours most of that formed during the day seems to have disappeared from the leaf. It is, in fact, changed to sugar prior to transportation but may be removed to other organs of the plant, as, for example, to fleshy roots or tubers, where it may again be converted into starch, accumulating at times to a very considerable extent.

Photosynthesis is most rapid under those conditions of temperature which are favorable for growth. Under strong light and favorable temperature, however, a slight increase in the amount of CO2 gives a higher rate of starch-production. The presence in the leaf or stem of other color bodies, such as browns and reds, is no indication that chlorophyl is absent. As a matter of fact, chlorophyl is generally present in such cases, but may be veiled by the more prominent color. In showy flowers, however, chlorophyl seldom occurs. Photosynthesis is inhibited by any condition affecting the general health of the plant, and it is low during cold and dark weather. The larger number of plants are most active in the brightest sunlight, but certain shade-loving species are injured by such exposures, and are adjusted to conditions of half-shade, such as obtain in the shade of trees or bushes. B.M.Duggar

The above text is from the Standard Cyclopedia of Horticulture. It may be out of date, but still contains valuable and interesting information which can be incorporated into the remainder of the article. Click on "Collapse" in the header to hide this text.

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